Conventionally, there is known an opening/closing valve including a plurality of butterfly valves, and shaft portions for connecting the butterfly valves to each other. For instance, Patent Literature 1 discloses an opening/closing valve including three butterfly valves disposed in an intake system of an internal combustion engine, and shaft portions for connecting the butterfly valves to each other. Axially projecting tubular-shaped shaft portions are integrally formed on opposite ends of the opening/closing valve. Supporting the shaft portions on opposite ends of the opening/closing valve by bearings makes it possible to rotatably support the opening/closing valve.
As disclosed in Patent Literature 1, when the opening/closing valve is provided to stride over a plurality of passages in an intake system of an engine, the butterfly valves or the shaft portions may cause plastic deformation as a result of application of intake pressure. This may make it difficult to perform smooth opening/closing operations and may make it difficult to secure the reliability of the opening/closing valve. When the opening/closing valve is provided in an exhaust system of the engine, thermal deformation due to heat of exhaust gas may be generated in addition to the above. This may cause another drawback.
Patent Literature 1 describes differentiating the directions in which the shaft portions for connecting the butterfly valves are likely to bend. However, Patent Literature 1 takes into consideration a problem on dimensional precision, which is involved when butterfly valves and shaft portions are integrally molded of resin. Specifically, warp or deflection is likely to occur when resin is molded. A variation in the size or shape of shaft portions accompanied by warp or deflection may cause an increase in the sliding resistance. The measure disclosed, in Patent Literature 1 is for the purpose of preventing the aforementioned drawback (i.e. for the purpose of suppressing an increase in the sliding resistance), and is not a measure for eliminating the aforementioned drawback i.e. deformation of butterfly valves or shaft portions by application of heat or pressure.